Under certain circumstances, the telescopic actuators used for moving one element relative to another element can be used in parallel with a member that imposes a distance between the two elements that are coupled to the actuator. This applies for example to an aircraft undercarriage having a leg that is hinged to the structure of the aircraft when it reaches a deployed position. It is known to couple a strut between the leg and the structure of the aircraft, the strut being locked in position when the leg reaches the deployed position, thereby stabilizing the leg in said position. The actuator for operating the undercarriage, which is also coupled to the leg of the undercarriage and to the structure of the aircraft, is then in parallel with the strut, which strut is rigid. The length of the actuator, i.e. the distance between its two attachment points, is then imposed by the strut. Furthermore, the leg and the strut may deform in service, thereby imposing variations in length on the actuator.
The hydraulic actuators that have been used in the past for raising undercarriages are well adapted to the raising function since they are reversible, thereby enabling them to follow the movements as imposed by deformation of the undercarriage without difficulty. Providing both chambers of the actuator are connected to the return, the opposing force delivered by the actuator against said variations of length is minimal.
Electromechanical actuators give rise to greater problems. In general, they are not reversible, which means that they cannot lengthen or shorten under the effect of external stress. Furthermore, they are liable to impose an opposing force that is very high. This opposing force stresses both the internal structure of the actuator and the points on the undercarriage or of the aircraft structure where it is attached.
Telescopic electromechanical actuators are also known that include a cylinder in which a rod is mounted to slide. The actuator has a lead screw that is actuated by an electric motor and that extends inside the rod. The rod of the actuator is secured to a nut that cooperates with the screw so that when the screw is rotated by the motor, the rod slides telescopically in the cylinder. The actuator includes retention means that hold a bearing for the screw in an axial direction. In certain actuators, the retention means are capable of releasing the bearing axially so as to allow the rod to move under the action of a load coupled to the rod, e.g. in the event of the motor jamming. That type of actuator is particularly adapted to the function of operating an undercarriage, since in the event of the motor jamming, it is still possible to release the rod, and thus to allow the undercarriage leg to be lowered down to its deployed position, under the effect of gravity. Nevertheless, in normal operation, that type of actuator is not reversible and suffers from the above-mentioned drawbacks.